1. Field of the Invention
The present invention relates to a semiconductor device and a manufacture method thereof, and in particular to a semiconductor device having a nonvolatile memory using a phase-change material and a manufacture method thereof.
2. Description of the Related Art
Semiconductor memories for use in semiconductor devices include volatile memories which lose stored information when power is removed, and nonvolatile memories which retain information even when power is removed. The volatile memories include a dynamic random access memory (DRAM) and a static random access memory (SRAM), while the nonvolatile memories include an electrically erasable programmable read only memory (EEPROM) and a flash memory. Most of recent portable information terminal devices employ a flash memory which retains stored information when power is removed, for the purpose of size reduction and power saving.
Recently, however, phase-change memories using a phase-change material have become to attract attention in order to pursue further size reduction and power saving. A phase-change memory is a nonvolatile memory which utilizes two different reversible phase changes of a phase-change material to store information. The phase-change material is changed to either the amorphous state with high resistance or to the crystalline state with low resistance to thereby store information of “1” or “0”. A chalcogenide material is used as such phase-change material
The rewrite of a phase-change memory is performed by supplying sufficient Joule heat to the phase-change material to once melt the same, and then quenching the molten material to the amorphous state (reset state) with high resistance. The phase-change material is converted to the crystalline state (set state) with low resistance by supplying slightly less Joule heat thereto and then annealing the same. The quantity of supplied heat and the cooling rate are controlled by the current value and length (duration) of a pulse applied to the phase-change material. The rewrite of the memory is thus performed by converting the phase-change material to the amorphous state or the crystalline state to change the resistance thereof. The read of the phase-change memory is performed by utilizing the fact that the current value is different between the amorphous state and the crystalline state of the phase-change material.
FIG. 1 is a partial cross-sectional view showing a principal part of a conventional phase-change memory cell. A lower electrode 6 is covered with an interlayer insulating film 5, which is formed with a contact hole. A heater electrode 1 is formed in the contact hole. The top surface of the heater electrode 1 is made flush with the top surface of the interlayer insulating film 5, and a phase-change film 2 and an upper electrode 3 are formed on the top surface thereof. The interlayer insulating film 5 is for example a silicon oxide film (SiO2). One end of the heater electrode 1 is connected to the lower electrode 6 while the other end is connected to the phase-change film 2. The phase-change film 2 is connected to the upper electrode 3,
The heater electrode 1 generates heat due to Joule heat evolved when voltage is applied between the lower electrode 6 and the upper electrode 3, whereby the crystal phase of the phase-change film 2 is changed. The change of the crystal phase of the phase-change film 2 changes the electrical resistance of the phase-change film 2. A region in which the crystal phase of the phase-change film 2 is changed is indicated as a phase-change region 4. A temperature of about 600° C. or higher is required to change the crystal phase of the phase-change film 2. Thus, the phase-change region 4 is defined by a region in the phase-change film 2 on and surrounding the contact surface between the heater electrode 1 and the phase-change film 2. The heater electrode 1 is formed of a material having an appropriate resistance, such as titanium silicon nitride, or tantalum nitride.
In the conventional phase-change memory, however, the phase-change film 2 is formed flat on the flat top surface of the heater electrode 1. Therefore, as indicated by the arrows in FIG. 2, the phase-change region 4 is also extended laterally, and heat is diffused to the upper side. When the phase-change region is extended and heat is diffused in this manner, heat generated by the heater electrode cannot be concentrated exclusively to the small phase-change region only, resulting in low efficiency of heat generation. It is therefore a crucial problem for the phase-change memories to develop a phase-change memory cell having a heater electrode and a phase-change film, which has a small phase-change region and is capable of efficient heat generation with a small amount of current.
The following patent documents are known as prior art documents relating to such phase-change memories. Japanese Kokai Patent Publication No. 2005-244235 (Patent Document 1) discloses a technique in which a spacer and a heater electrode are formed within a contact hole formed through an interlayer insulating film so that the heater electrode is in contact with a flat phase-change film. Japanese Kohyo Patent Publication No. 2006-510219 (Patent Document 2) discloses a technique in which a contact hole is formed through an interlayer insulating film on a heater electrode, and a spacer and a phase-change film are formed within the contact hole. Japanese Kohyo Patent Publication No. 2006-510218 (Patent Document 3) discloses a technique in which a conductor and a heater electrode are formed within a contact hole through an interlayer insulating film, and the heater electrode is in contact with a flat phase-change film.